1. Technical Field
The invention is related to fabrication of microelectronic integrated circuits with a inductively coupled RF plasma reactor and particularly to such reactors having coiled RF antennas providing a highly uniform plasma distribution.
2. Background Art
Inductively coupled plasma reactors are employed where high density inductively coupled plasmas are desired for processing semiconductor wafers. Such processing may be etching, chemical vapor deposition and so forth. Inductively coupled reactors typically employ a coiled antenna wound around or near a portion of the reactor chamber and connected to an RF power source. In order to provide a uniform etch rate or deposition rate across the entire surface of a wafer, the plasma density provided by the coiled antenna must be *uniform across the surface of the semiconductor wafer. One attempt to provide such a uniform field is to wind the coiled antenna in a flat disk parallel to overlying the wafer, as disclosed in U.S. Pat. No. 4,948,458 to Ogle. This concept is depicted in FIG. 1.
One problem with the flat coiled antenna of FIG. 1 is that there is a large potential difference between the center of the antenna and the circumferential edge thereof, with the result that the plasma can have a high ion density or xe2x80x9chot spotxe2x80x9d over the center of the wafer and a lower ion density at the wafer periphery. This in turn causes the etch ratexe2x80x94or deposition ratexe2x80x94to be nonuniform across the wafer surface. One way of ameliorating this problem is to limit the power applied to the antenna coil to a few hundred watts so as to minimize the plasma non-uniformity. This approach is not completely satisfactory because it limits the etch rate (or deposition rate), thereby reducing throughput or productivity of the reactor, and moreover does not solve the problem of process non-uniformity across the wafer surface.
Another problem with inductively coupled reactors is that any high voltage applied to the antenna coil leads to capacitive coupling of RF power to the plasma. In other words, capacitive coupling of RF power from the coiled antenna to the plasma increases with the voltage on the coiled antenna. Such capacitive coupling can increase the ion kinetic energy which makes it difficult for the user to precisely control ion kinetic energy and thereby control sputtering rate or etch rate. Capacitive coupling is particularly pronounced in the flat disk coil antenna of FIG. 1.
Therefore, there is a need for an inductively coupled plasma reactor having a coiled antenna which provides a highly uniform plasma across the wafer surface at high power with only minimal capacitive coupling.
The invention is embodied in a coil antenna for radiating RF power supplied by an RF source into a vacuum chamber including a plurality of conductors wound about the axis of symmetry as respective concentric helices, the respective helices being laterally displaced from the axis of symmetry in generally the same manner, the respective helices being offset from one another in the direction generally of the axis of symmetry, each of the conductors being adapted from connection across a supply of RF source power.